A person doesn’t have to be suffering from mental illness to see a therapist and derive value from it. The D may be depressed or anxious. But if she’s not, which is also quite possible, having a therapist who can help her navigate her feelings about what is going on in her life could be helpful. It does sound like the D could benefit from that.
For the same reason, the OP could probably benefit from having her own therapist, which has been suggested. The weight of worrying about a child is enormous on its own. The OP has expressed concern that discipline is at odds with love and a healthy relationship with her D. That’s such a normal and relatable parental tension, and a therapist can also help her understand what and why she’s feeling and how to process that confusion and her feelings around actions she takes.
Finding a good therapist is hard. Doing the work in therapy once you have can also be hard. But almost certainly worth it.